Prior art microprocessors controlled by a control program have recently been used for data entry terminals, retail sales transaction terminals, general calculators and the like. Normally, most such units have been tied to large central computers for the storage of data, e.g., for inventory control, for credit verification, etc. However, none of the prior art terminals or calculators have provided the ability for step by step enabling of the filling out of forms including the calculations associated with each step on the form. Nor have such systems provided means for updating certain portions of the control program associated with calculator operation such that as the required steps associated with each form changes, the apparatus can also be changed to accommodate them.